The present invention relates to expandable endoprosthesis devices, generally called stents, which are adapted to be implanted into a patient""s body lumen, such as a blood vessel, to maintain the patency thereof Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis.
Stents are generally cylindrically- shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or super-elastic nickel-titanum (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery catheter into the blood vessel. Such stents manufactured from expandable heat sensitive materials allow for phase transformations of the material to occur, resulting in the expansion and contraction of the stent.
Details of prior art expandable stents can be found in U.S. Pat. No. 3,868,956 (Alfidi et al.); U.S. Pat. No. 4,512,1338 (Balko et al.); U.S. Pat. No. 4,553,545 (Maas, et al.); U.S. Pat. No. 4,733,665 (Palmaz); U.S. Pat. No. 4,762,128 (Rosenbluth); U.S. Pat. No. 4,800,882 (Gianturco); U.S. Pat. No. 5,514,154 (Lau, et al.); U.S. Pat. No. 5,421,955 (Lau et al.); U.S. Pat. No. 5,603,721 (Lau et al.); U.S. Pat. No. 4,655,772 (Wallstent); U.S. Pat. No. 4,739,762 (Palmaz); and U.S. Pat. No. 5,569,295 (Lam), which are hereby incorporated by reference.
Further details of prior art self-expanding stents can be found in U.S. Pat. No. 4,580,568 (Gianturco); and U.S. Pat. No. 4,830,003 (Wolff, et al.), which are hereby incorporated by reference.
Expandable stents are delivered to the target site by delivery systems which often use balloon catheters as the means for delivering and expanding the stent in the target area. One such stent delivery system is disclosed in U.S. Pat. No. 5,158,548 to Lau et al. Such a stent delivery system has an expandable stent in a contracted condition placed on an expandable member, such as an inflatable balloon, disposed on the distal portion of an elongated catheter body. A guide wire extends through an inner lumen within the elongated catheter body and out its distal end. A tubular protective sheath is secured by its distal end to the portion of the guide wire which extends out of the distal end of the catheter body and fits over the stent mounted on the expandable member on the distal end of the catheter body.
Some prior art stent delivery systems for implanting self-expanding stents include an inner lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner lumen to xe2x80x9cuncoverxe2x80x9d the compressed stent, allowing the stent to move to its expanded condition into the target area.
In many procedures which utilize stents to maintain the patency of the patient""s body lumen, the size of the body lumen can be quite small which prevents the use of some commercial stents which have profiles which are entirely too large to reach the small vessel. In particular, often in PTCA procedures, the stenosis is located in the very distal regions of the coronary arteries which often have small diameters. Many of these distal lesions are located deep within the tortuous vasculature of the patient which requires the stent to not only have a small profile, but also high flexibility to be advanced into these regions. As a result, the stent must be sufficiently flexible along its longitudinal axis, yet be configured to expand radially to provide sufficient strength and stability to maintain the patency of the body lumen. Since many commercial stents lack both the low profile and extreme flexibility needed to reach such distal lesions, they are not available for utilization for such procedures.
What has been needed is a stent which has a low profile and a high degree of flexibility so that it can be advanced through tortuous passage ways of the anatomy and can be expanded within the body vessel to maintain the patency of the vessel. Additionally, the expanded stent must have adequate structural strength (hoop strength) to hold the body lumen open once expanded. Such a stent should also have sufficient radiopaque properties to permit it to be sufficiently visualized on external monitoring equipment, such as a fluoroscope, to allow the physician to place the stent in the exact target location. The present invention satisfies these and other needs.
The present invention is directed to stents having low profiles which can be used in body vessels, such as the carotid arteries and other peripheral arteries, along with the coronary arteries. The stents of the present invention are intended, but are not limited, to the effective treatment of diseased vessels having diameters from about 3.0 to 14.0 millimeters.
The stents of the present invention can be formed from super elastic nickel titanium alloys, or other shape memory materials, which allow the stent to be self expandable. The expansion occurs when the stress of compression is removed. This allows the phase transformation from martensite to austenite to occur, and as a result the stent expands. The stents of the present invention can be processed to behave superelastically at body temperature. Alternatively, the stent designs of the present invention could be used in conjunction with balloon expandable stents made from stainless steel or other conventional stent materials.
In all embodiments, the stents of the present invention have sufficient longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens, yet remain stable when expanded radially to maintain the patency of a body lumen, such as an artery or other vessel, when implanted therein. The present invention particularly relates to unique strut patterns which have a high degree of longitudinal flexibility and conformability, while providing sufficient radial-expansibility and strength to hold open the body lumens. The high radial strength possessed by the stents of the present invention allow them to be used in treating calcified lesions.
Generally, the greater the longitudinal flexibility of the stents, the easier and the more safely they can be delivered to the implantation site, particularly where the implantation site is on a curved section of a body lumen, such as a coronary artery or peripheral blood vessel, and especially in saphenous veins and larger vessels. The designs of the present invention have sufficient flexibility to conform to the patient""s vasculature, thus preventing vessel straightening by the stent. Moreover, the stents of the present invention are crush proof, making them particularly suitable for implantation in the carotid arteries.
Each of the different embodiments of stents of the present invention include a plurality of adjacent cylindrical elements (often referred to as xe2x80x9cringsxe2x80x9d) which are generally expandable in the radial direction and arranged in alignment along a longitudinal stent axis. The cylindrical elements are formed in a variety of serpentine wave patterns transverse to the longitudinal axis and contain a plurality of alternating peaks and valleys. At least one interconnecting member (often referred to as a xe2x80x9cspinexe2x80x9d) extends between adjacent cylindrical elements and connects them to one another. These interconnecting members insure minimal longitudinal contraction during radial expansion of the stent in the body vessel. The serpentine patterns have varying degrees of curvature in the regions of peaks and valleys and are adapted so that radial expansion of the cylindrical elements are generally uniform around their circumferences during expansion of the stent from the collapsed position to the expanded position.
The stents of the present invention also have strut patterns which enhance the strength of the ends of the stent and the overall radiopacity of the stent, yet retain high longitudinal flexibility along their longitudinal axis to facilitate delivery through tortuous body lumens and remain stable when expanded radially to maintain the patency of the body lumen. The present invention in particular relates to stents with unique end portions having sufficient hoop strength to maintain a constant inner diameter which prevents the stent from taking on a xe2x80x9ccigarxe2x80x9d shape when deployed in the body lumen. The end rings used with the present invention are particularly useful on self-expanding stents which may otherwise have end rings that are more susceptible to compressive forces.
The resulting stent structures are a series of radially expandable cylindrical elements that are spaced longitudinally close enough so that small dissections in the wall of a body lumen may be pressed back into position against the luminal wall, but not so close as to compromise the longitudinal flexibility of the stent both when negotiating through the body lumens in their unexpanded state and when expanded into position within the vessel. The design of the stents contribute to form small gaps between struts to minimize tissue prolapse. Each of the individual cylindrical elements may rotate slightly relative to their adjacent cylindrical elements without significant deformation, cumulatively providing stents which are flexible along their length and about their longitudinal axis, but which still are very stable in their radial direction in order to resist collapse after expansion.
In one embodiment of the present invention, each cylindrical element of the stent includes eight peak regions (often referred to as xe2x80x9ccrownsxe2x80x9d) and eight valley regions which provide sufficient coverage of the vessel when placed in the expanded or deployed position. In this design, each cylindrical element consisting of an alternating pattern of U-shaped portions and double-curved (W) portions connected both axially and circumferentially to eight discontinuous interconnecting members or spines. For example, the U-shaped portion of the cylindrical element can be connected to an adjacent cylindrical element via the interconnecting members. The same cylindrical element can be then connected to another cylindrical element via the interconnecting members connected to the double-curved portions. The cylindrical element can be connected to an adjacent cylindrical element by four interconnecting members. This particular alignment of interconnecting members provides adequate flexibility to the stent and also helps prevent foreshortening of the stent as it expands radially outward. The discontinuing pattern of interconnecting members results in a highly flexible stent that does not kink upon bending. Both the distal and proximal ends of this stent design can be entirely composed of xe2x80x9cWxe2x80x9d patterns which provide additional strength to the ends of the stent. The resulting stent produces an eight crown, four-cell pattern which has sufficient coverage for vessel scaffolding while maintaining excellent flexibility to reach distal lesions and possessing sufficient radial strength to hold the target vessel open. An alternative pattern using six crowns and six discontinuous interconnecting members also can be utilized and will exhibit these same physical properties.
The serpentine pattern of the individual cylindrical elements can optionally be in phase with each other in order to reduce the contraction of the stent along their length when expanded. In these embodiments of the present invention, interconnecting members align behind each other to create a continuous xe2x80x9cspinexe2x80x9d which extends from one end of the stent to the other. Two or three rows of continuous spines can be used to connect adjacent cylindrical elements. This construction also helps prevent the stent from foreshortening when expanded.
A stent made in accordance with the present invention can be readily delivered to the desired target location by mounting it on a stent delivery catheter which includes a retractable sheath, or other means, to hold the stent in its collapsed position prior to deployment.
These and other features and advantages of the present invention will become more apparent from the following detailed description of the invention, when taken in conjunction with the accompanying exemplary drawings.